1. Field of the Invention
The present invention relates to a developing unit.
2. Background Information
An electrophotographic image forming device such as a laser printer or a copying machine comprises a photosensitive drum and a developing unit that provides toner to the surface of the photosensitive drum. In addition, the developing unit includes a developing roller that supports developer on the surface thereof, and an agitation transport member that agitates the developer while simultaneously transporting it in the axial direction and providing it to the developing roller. Furthermore, the developing roller generally includes a plurality of magnetic poles, including a drawing pole that draws developer onto the surface of the developing roller and a peel-off pole that peels off the developer remaining on the surface of the developing roller after passing by a developing region in which toner is supplied to the photosensitive drum from the surface of the developing roller.
Japan Patent Application Publication JP-A-04-060665 discloses the above described type of developing unit, which includes a first agitation transport member and a second agitation transport member, which are arranged parallel to each other in a horizontal direction, and a developing roller arranged above the first and the second agitation transport members. According to this developing unit, the two agitation transport members are arranged parallel to each other. Therefore, it is possible to manufacture the entire developing unit to be slim. In addition, according to this developing unit, it is possible to arrange the developing unit below the photosensitive drum. Thus, it is possible to miniaturize the image forming device in the lateral direction.
However, in this developing unit, the developer contained in the first agitation transport member side is drawn to the developing roller and transported to the developing region. The remaining developer is peeled off the surface of the developing roller after passing the developing region, and dropped on the second agitation transport member side. As a result, the amount of the developer increases on the second agitation transport member side. On the other hand, the amount of the developer on the first agitation transport member side decreases. Thus, the amount of the developer contained in the two sides becomes imbalanced. Accordingly, the density of the developed image may appear to be weak, or the developer may overflow out of the second agitation transport member side.
In order to resolve the above described problem, Japan Patent Application Publication JP-A-10-123816 discloses a developing unit in which the amount of the developer contained on the first agitation transport member side and the second agitation transport member side can be balanced. In this developing unit, a larger quantity of developer contained in the second agitation transport member side will be moved to the first agitation transport members side by taking one of the following measures, for instance. One measure is forming a sloping base on a housing so that the first agitation transport member side can be arranged below the second agitation transport member side. The other measure is adjusting the amount of the developer transported between these two agitation transport members by changing the size of an opening formed on the partition plate that is arranged between these two agitation transport members in the horizontal direction.
However, if the base of the housing is formed in a slope, the entire first agitation transport member will be drowned by the developer. Thus, agitating and transporting the upper portion of the developer away from the agitation transport member will be extremely difficult. On the other hand, enlarging the openings on the partition plate will not effectively balance the amount of developing agent on the two agitation transport member sides, because it will be time consuming. Therefore, this solution will only be sufficient for a low speed image forming device that outputs a few number of print sheets at a time.
On the other hand, Japan Patent Application Publication JP-A-2002-148915 discloses a developing unit in which the developing roller is arranged diagonally above the first agitation transport member and facing the second agitation transport member. More specifically, in this developing unit, the first agitation transport member and the second agitation transport member are arranged to be parallel to each other and separated by a partition plate which has openings on both ends in the axial direction. Here, the developer is drawn from the first agitation transport member side onto the developing roller that is arranged diagonally above the first agitation transport member. In addition, the developer will be peeled off from the surface of the developing roller after passing the developing region, and returned back to the first agitation transport member side.
However, if a structure in which four developing units corresponding to four color developers, i.e., the yellow (Y) developer, the magenta (M) developer, the cyan (C) developer, and the black (K) developer, are respectively arranged parallel to each other in the horizontal direction, just like a tandem full color printer, the horizontal size thereof will be increased. Therefore, it is difficult to miniaturize the developing unit. On the other hand, if the horizontal size of the image forming device needs to be reduced, the arrangement of the developing roller has to be closely similar to that described in Japan Patent Application Publication JP-A-10-123816. In this case, the above described problem regarding the invention described in the publication will be caused.
A portion of the developer peeled off from the developing roller may be magnetically attracted to the developing roller without being agitated by the agitation transport member, and thus directly drawn to the surface of the developing roller. As a result, an excessive amount of charge will be applied to the developer. Thus, the amount of the developer supplied to the photosensitive drum will decrease, and the developed image will appear to be pale in color.
In addition, the developing unit disclosed in Japan Patent Application Publication JP-A-2002-148915 has a large width (i.e., large horizontal length). Because of this, an image forming device, such as a tandem color printer in which a plurality of the developing units are arranged parallel to each other in the horizontal direction, will have a large horizontal size.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved developing unit. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.